Such a transmission is generally known, for example from the European patent publication EP-A-0777069 or from SAE technical paper series 881734. In the known transmission, with the application of axial force the so-called push-belt is clamped between a pair of slightly conical sheaves of two pulleys set up at a distance from each other. The belt and a sheave rest against each other in a contact face which forms an acute angle, the flank angle or cone angle, with an orthogonal on the axis of rotation of the pulley. The clamping force of the two pulleys is such that a state of equilibrium with a fixed transmission ratio arises. The transmission proceeds to continuously variable changing of this ratio, i.e. stageless shifting while increasing the clamping force in one of the two pulleys. In the known transmission the push-belt is composed of at least one flexible band assembly, which is generally formed from metal rings and is capable of taking pulling force, and of transverse elements which at least largely freely surround the band assembly and enable the belt to transmit pushing force. The generally known transmission is usually provided with a flank angle of 11 degrees or 0.192 radians.
A wedging action between belt and pulley is associated with the acute flank angle. In order to permit shifting of the transmission during standstill, according to a generally accepted rule, the tangent of the angle of the contact face must be greater than the coefficient of friction, so that the belt can be moved in the radial direction, or tan .lambda.&gt;.mu.. The line with points in which tan .lambda.&gt;.mu. is known as the self-locking limit. Close to this self-locking limit, in order to permit movement of the belt or shifting, the axial force of one of the two pulleys of a transmission must be very much greater than the axial force acting upon the other pulley. In a practical application of a transmission, in particular a vehicle transmission, the capacity of the power generator present for this purpose may be inadequate in this case. Therefore, in the design of a continuously variable transmission the self-locking limit constitutes a test parameter as regards the ratio between the flank angle and the coefficient of friction in the radial direction.
However, it has been found in practical tests that transmissions provided with a push-belt of the type described above can produce unexpected breakage of the belt, even if the flank angle meets the limit set by self-locking. The object of the present invention is to provide a transmission with a belt of the abovementioned type, in which the ratio between flank angle and coefficient of friction is such that the durability of the construction is retained, in other words, belt breakage does not occur or is very unlikely to occur in the short or even longer term.